The present invention relates to a bonded type piezoelectric apparatus composed of a piezoelectric ceramic element bonded with a mounting member into one body and a method of manufacturing the same and, more particularly, to a ceramic piezoelectric apparatus which is suitable to sensors for detecting an elastic wave and various actuators and a method of manufacturing the same.
Elements for converting an electric quantity into a mechanical quantity, namely, what is called transducers, of various system are known and used as sensors and various actuators. One of them is a piezoelectric type transducer and, in particular, a transducer utilizing the piezoelectric characteristic of a piezoelectric ceramic, namely, a ceramic piezoelectric apparatus is widely used.
In some of such conventional bonded type piezoelectric apparatuses, a synthetic resin adhesive is used in manufacturing them. Such piezoelectric apparatuses are only usable in a temperature range of, for example, -40.degree. C. to 80.degree. C.
Recently, however, non-destructive testing techniques such as an acoustic emission (AE) method, an ultrasonic flaw detecting (UT) method and a vibration method have been developed. In the case of adopting these technique, the ambient temperature is too high for the above-described conventional bonded type piezoelectric apparatuses.
Further, in an application for monitoring a superconductivity apparatus, such is required to be used in an extremely low temperature, the conventional ceramic piezoelectric apparatus could not be used.
When a sensor is applied to the AE method, it is necessary to transmit the elastic wave supplied from the outside to a sensor element with efficiency and to have a sufficient sensitivity as a sensor, so that what is called a bonded type piezoelectric sensor is necessary, which is composed of a sensor element of a piezoelectric ceramic material with a bonded surface thereof bonded with a mounting member into one body. The above-described conventional bonded type piezoelectric apparatuses, however, are difficult to apply to the sensor, because a synthetic resin adhesive is used for bonding, which greatly attenuates the elastic wave.
As a countermeasure, a method of bonding these members by soldering has been proposed in, for example, Japanese Patent Application Laid-Open Nos. 52-19094 (1977), 58-99100 (1983) and 57-201399 (1982). Among the above three prior arts, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 52-19094 (1977), piezoelectric element are directly soldered to a metal body at a temperature of not higher than the Curie temperature (190.degree. C.) of the piezoelectric element in order to enhance the efficiency of the sensor element, and in the apparatus disclosed in Japanese Patent Application Laid-Open No. 58-99100 (1983), the polarized piezoelectric element is directly bonded by a solder having a low melting point (143.degree. C.) in order to shorten the bonding time, and further, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 57-201399 (1982), because the polarization is destroyed during a high temperature treatment, therefore, for preventing the same, before inducing the polarization, an electrode face formed by metallizing (usually carried out with Ag) the bonding surface of the piezoelectric element and an acoustic matching layer are bonded and the polarization treatment is carried out.
Accordingly, these apparatus can be used at temperatures upto the melting point of the solder which is higher than the above mentioned temperature 80.degree. C., however, the members are merely and directly bonded or the metallizing layer is merely formed with Ag, there was a problem that the bonded portion is likely to peel off at high or low temperatures during the use. In addition, since the thermal expansion coefficient of the piezoelectric ceramic is not taken into any consideration, the thermal strain caused during soldering is generated as a residual stress, which brings about problems such as the separation of the bonded portion and the breakage of the piezoelectric element during the manufacturing process or use.